1. Field of the Invention
This invention relates to the treatment of subterranean formations. In particular, it relates to the acid treatment of subterranean formations surrounding oil wells, gas wells and similar boreholes.
2. Description of the Prior Art
Acid solutions are often used to stimulate the production of fluids from limestones, dolomites, sandstones and other calcareous and siliceous formations surrounding oil and gas wells. The methods employed at pressures below fracture pressures are referred to as "matrix acidizing" methods. These methods involve the injection of an acid solution capable of reacting with constituents in the formation. The constituents may be a part of the formation itself (e.g., limestone or sandstone rock) or may be present as formation "fines". Material passing a No. 400 U.S. mesh (&lt;38.mu.) is usually referred to as fines. In addition to the well-known clays, formation fines may include materials such as quartz, dolomite, calcite and amorphous solids. Drilling fluid solids such as bentonite and barite may also be present due to earlier drilling fluid invasion.
It has long been known that formation fines can drastically reduce production. They tend to migrate and collect and restrict flow passages, or completely plug formation pores. The reduction in permeability caused by fines lodging in the pores in the critical flow region immediately surrounding the wellbore is particularly serious.
Fines plugging may involve several mechanisms including particle bridging, capillarity and mixed wettability phenomena. Bridging of fines can plug a pore several times larger than the average particle size of the fines. Once a bridge is formed, the fines become tightly packed as a result of differential flow pressure. The amorphous component of the fines may add a degree of cementation to the mass.
Perhaps less understood is the role of capillarity and the wettability of the fines. Sandstones and limestones tend to be predominantly water-wet so that water-wet fines lodged in pore spaces are immersed in the wetting phase. However, oil-wet spots may be present on the matrix proper and within the agglomerated fines so that some crude oil may be adhered thereto to further add to the complexity of the situation.
These factors frequently prevent the acid solution from contacting and reacting intimately with the fines. Only the outer surface of the packed fines is exposed. Although large volumes of acid may be used to dissolve the fines, acid treatments are frequently unsuccessful because the acid is incapable of reaching and reacting intimately with the material that plugs the pores. Since fines agglomerations are saturated with formation brine, dissolution of fines by acid depends on diffusion of acid into the agglomeration, and this is a very slow process.
Several prior matrix acidizing techniques have been developed, but none attack the problem of dislodging fines from the pores to permit intimate mixture with the acid solution.
Gidley (U.S. Pat. No. 3,481,404) teaches the injection of a mutual solvent, after the injection of an acid solution, to retard the formation of emulsions and promote water-wetting of the formation materials.
Clampitt (U.S. Pat. No. 4,007,789) employs alternate slugs of acid solutions and aqueous gels to improve the acidization of multiple zone formations.
Crowe (U.S. Pat. No. 3,962,102) contemplates the use of an acid-in-oil emulsion for the purpose of extending the reaction time of the acid.
Gogarty (U.S. Pat. No. 3,568,772) discloses the use of a micellar dispersion (microemulsion) as a preflush for the acid to condition the formation.